Draft control system



Gd. 1942- G. E. STOLTZ 2,297,812

DRAFT CONTROL SY STEM Filed June 50, 1959 I 5 Sheets-Sheet 1 K x KR a sH WiTNESSES: XNVENTOR Oct. .6, 1942. G. E. STOLTZ I DRAFT CONTROL SYSTEM3 Sheets-Sheet 2 Filed June 30, 1959 INVENTOR Gi /227 ESza/zz.

a. E. STOLTZ DRAFT CONTROL SYS'I'H Filed June so, 1939 :s Sheds-Sheet sQE xrmQm 2 4 3 I, r 0 w, w S m 6 3\ W .m 42M. 4? 2 mm mam 5 M 0 Z 0 n 1Pe z n a e P wnl p l w lx 4 3 2 1 KE xRMQLM m m. m

.i Percentflraft- INVENTOR G/erznEfiYo/tz.

WITNESSES:

Patented Oct. 6, 1942 DRAFT CONTROL SYSTEM Glenn E. Stolts, Pittsburgh,Pa., assimor to Westinzliouse Electric & Manufacturing Company, EastPittsburgh, Pa a corporation of Pennsyl- Vania Application June 30,1939, Serial No. 282,197

13 Claims.

My invention relates, generally, to draft control systems for rollingmills, and, more particularly, to systems for controlling and indicatingthe draft taken on strip material as it passes through a rolling mill,

In the operation of rolling mills, and particularly strip rolling mills,it is necessary that the amount of reduction of the material beingrolled, commonly known as the draft, be indicated and very closelycontrolled so that the rolled product will have a uniform thickness orgauge.

An object of my invention is to provide a draft control system whichshall function to indicate and control the drait taken on a length ofmaterial as it passes through the reducing rolls of a rolling mill. Y

A further object of my invention is to provide a draft control systemfor rolling mill which shall function to accurately indicate and controlthe draft taken by the reducing rolls, which shall be adjustable to takeany desired predetermined draft and which shall automatically compensatefor variations in the mill speed.

Another object of the invention is to provide a draft indicoting andcontrol system which shall be simple and emcient in operation and whichshall he inexpensive to manufacture, install, and maintain These andother objects and advantages of the invention will be apparent from thefollowing detailed description taken in connection with the accompanyingdrawings in which like reference characters designate identical orsimilar elements and in which:

Figure l is a diagrammatic view of a strip rolling mi l and draftcontrol means therefor embodying the principal features oi a preferredembodiment of my invention;

Fig. 2 is a diagrammatic view of a strip rolling mill and draft controlmeans therefor embodying the principal features of another embodiment ofmy invention; and

Figs. 3 and i are graphs illustrating the operating characteristics ofcertain parts of the embodiment of the invention shown in Fig. 2.

In practicing the preferred embodiment oi my invention I provide a pairof rollers l6 and I! which bear upon the strip I4 on opposite. sides ofthe reducing roll stand I as it passes between the reducing rolls andbetween the reels i8 and 20. The rollers l and 12 are driven by thestrip and their motion is transmitted to opposite sides of aspeed-differential responsive device 22. A variable-ratio transmissiondevice 24 is interposed between the roller l0 and its associated side ofthe speed-differential responsive device 22 to compensate for thedifference in the speeds of the strip on the opposite sides of the milldue to reduction of the strip in the mill.

A control device 26 responsive to the speeddiflerential responsivedevice 22 functions to control the screw-down actuating mechanism 28.

A stroboscopic device 39 iunctions to indicate any deviation of thedraft from that determined by the setting or the variable speed-ratiotransmission device 24. A series of contact devices 32 is provided forcalibrating the variable speedratio transmission device 24 and forverifying its calibration.

In the embodiment of the invention shown in Fig. 2, one side of thespeed-difierential responsive device 22 is driven in accordance with therotation of the roller and the other side is.

driven in accordance with the rotation oi the roller 32 through areduction gear unit 34. The speed difierential responsive device 22drives a generator 36 which is connected to energize a drai't indicator38 a regulating device to. The regulating device w functions to controlthe screw-down actuating mechanism 28 and a strip .tensionihg, device32. Variations in the speed oi the mill are compensated ior hy generatorit Whose output potential is made to vary, inversely with the of thestrip on the entry side or the mill. The generator 36? is driven at aspeed proportional to the difference between the speed of the roller 92and a constant speed motor 58 through a difierehtioi mechanism 8.

The graph of Fig. 3 indicates the sheet oi. the reduction gear unit 35in the system.

The curves of Fig. i represent the output potentials of the generators35 and 44 at the difierent strip speeds and the resultant potentialsacting on the indicator 38 and the regulator M1 at the different degrees01' reduction of the strip in the mill.

Referring again to Fig. i for a more complete description, the shaft 50of the variable speedratio transmission device 24 is driven by theroller it through a synchro-tie system 5'2 which has a transmittingelement 54 driven by roller iii and a receiving element 55 connected todrive the shaft 50. The roller Ii is mounted. upon a frame 58' which ismounted for a vertical movement against the bias or a spring element 80.A contact element 82 is mounted on the frame 58 and is positioned toengage fixed contact element 64 when the frame 58 is pushed downwardagainst the bias of this spring by the engagement of the strip H withthe roller ll.

In a similar manner a shaft 68 is driven by the roller I? through asynchro-tie system 68 and movable and fixed contact elements l8 and 12are positioned to engage when the roller l2 and its supporting frame aremoved downward against the biasing of a spring M by engagement of thestrip i l with the roller E2.

The variable speed-ratio transmission device 2 comprises a cone pulley35 mounted on a shaft 50 and a cone pulley it mounted on a shaft i8 anda belt all extending in driving relation between the cone pulleys. Theposition of the belt 85) along the pulleys determines the speed ratio ofthe pulleys and the positioning of the belt may be manually controlledby an adjusting wheel 82 which drives a screw dd. The screw 841 hasscrew-threaded engagement with a belt guide 86 which moveslongitudinally of the screw 36 upon actuation of the screw to thusadjust the position of the belt at with respect to the cone pulleys i land it. An indicator pointer 88 is mounted for movement with the beltguide 85 and cooperates with a scale 9% to indicate the adjustment ofthe transmisison device. Scale all maybe calibrated in terms of per centreduction so that the variable speed-ratio transmission device 2 3 maybe set to produce any desired reduction of the strip l l.

The driven shafts it and 6d are connected by means of electricallyoperated clutches 92 and 93 to drive the shafts 96 and ed. The shafts 96and 98 drive the opposite sides of the speed-differential responsivedevice 22 which operates a shaft are through the gear members m2 andN36. The control device 26 comprises an operating arm me which ismounted upon the shaft "it for rotation by the shaft through the arcbetween the contact elements H l and M6 and which is biased to a centralneutral position, as indicated, by means of spring members use and Mt. Acontact element H2 is mounted upon the arm MB to be moved by rotation ofthe arm 596 into engagement with fixed contact elements HQ and M6. Thecontact elements H2, H6 and H6 operate through the conductors M8 andIE9, as

indicated, to actuate the motor controller which in turn controlsactuation of and the direction of rotation of the armature 022 of thedrive motor for the screw-down actuating mechanism 28 through theconductors i2 3 and 026. r The motor controller indicated may be anysuitable controller and may take the form of the controller shown inFig. 7 of the patent to. Lamb No. 2,15%- 946.

The sets of cooperating contact elements 62-6l and id-l2 control theactuation of the .clutches 92 and 9 through a circuit which extends fromone side of a suitable source of electrical energy as indicated throughthe contact elements 62 and 66, the conductor I28, the contact elementare of a pushbutton switch 32, the conductor IN, the winding of theclutch 92, the conductor 936, the winding of the clutch 96, theconductor E38, and contact elements. i2 and it to the other side of thesource of energy.

The stroboscopic device 3@ comprises a disc Mil mounted for rotation bythe shaft '88 and an illuminating device M2 positioned to illuminate thedisc fl lil when energized. The illuminating device M2 may be anysuitable devicecommonly used for this purpose, such as a grid-glow tube.

A circuit interrupter M4 is mounted to be driven by the shaft 66 and isconnected to intermittent- 1y energize the illuminating device M2through a circuit which extends from one side of a suitable source ofelectrical energy through the illuminating device Hi2, a conductor Hit,a circuit control element M8, a conductor Edd, and the circuitinterrupter M4 to the other side -of the source of energy.

In the operation of the system, as the strip is passed through the milland extends between the reels l8 and 20,, it will bear upon the rollersid and E2 to close the control circuit for the clutches 92 and 9 5 thuscausing engagement of the clutches. The rollers i8 and I2 will be drivenat speeds proportional to the speeds of the strip. on the opposite sidesof the mill. The

speeds of the strip on the opposite sides of the mill will be difierent,and their ratio will depend on and will be proportional to the amount ofreduction of the strip M as it passes through the reducing rolls. Theadjusting wheel 82 will be manipulated to adjust the variablespeed-ratio transmission device 26 to produce a speed ratio of thedevice corresponding to the ratio of the speeds of the strip on the twosides of the mill which will exist when the desired amount of reductionin the mill is being produced. With the desired reduction being producedin the mill, it will be seen that the drive connection between theroller l2 and the shaft 98 which drives one side of thespeed-differential responsive device 22 and the drive connection betweenthe roller id and the shaft 96 which drives the other side of the speed-difierential responsive device 22 will produce rotation of the, shafts9t and 98 at equal speeds, and no rotation of the shaft iilii to actuatethe control device 2G will occur. If,

for any reason, a difierent degree of reduction of the strip it! thanthat corresponding to, the setting of the variable-speed-ratiotransmission device 26 is produced, there will be a difierence in thespeeds of rotation of the shafts 96 and 98 and this difierential motionwill be transmitted to the shaft 600 which in turn will cause thecontrol device 26 to actuate the screw-down actuating mechanism to soadjust the screw-down setting of the mill as to produce the desireddegree of reduction of the strip i i.

The stroboscopic device 3E will function to give a visual indication ofany variation of the degree of reduction of the strip from the desiredpredetermined amount. With the circuit control device 55.8 in the closedcircuit position, it will be seen that the illuminating device M2 willbe energized to intermittently illuminate the disc M0 at a frequencyproportional to the speed of the strip M on the right-hand side of themill. The disc Mil being driven by the shaft it will be rotating at aspeed corresponding to the speed of the strip on the left-hand side orthe mill mul tiplied by the speed ratio of the variable-speedratiotransmission device 26. If then the amount of reduction in the strip issuch that the ratio of the speeds of the strip on the opposite sides ofthe mill is the same as the ratio of the variablespeed-ratiotransmission device 2%, then the speed of rotation of'the disc Mil willcorrespond to the frequency of illumination of the disc use by theilluminating device M2, and the disc will appear to be standing still.However, if the degree of reduction of the strip in the mill is not suchas to produce a ratio between the speeds of the strip on the oppositesides of the mill corresponding to the speed ratio of thevariable-speedratio transmission device 24, the motion of the disc Milwill not correspond to the frequency of illumination of the disc by theilluminating de-v vice I42, and the disc I42 will appear to be moving.

The series of contact devices 22 provided for calibrating thevariable-speed-ratic transmission device 24 and for verifying itscalibration comprise circuit interrupters I52, I54 and I52 driven bygears I58-I50, I62-I54, and ISL-I52, respectively. The ratios betweenthe sets of gears I58-I60, IG2I64, lit-I82 have definite ilxed values.The interrupter I52 may be connected to energize the illuminating deviceI42 through a circuit which extends from one side of a suitable sourceof electrical energy through a conductor I10, the interrupter I52, theconductor I12, the circuit-control device I14, the conductor I46, andthe illuminating device I42 to the other side of the source of energy.The interrupters I52 and I54 may be connected to control theenergization of the illuminating device I42 in a similar manner by meansof the selectively actuable-circuit-control device I16 and I12.

The sets of gears may be driven by a. shaft I80 which may be connectedto the shaft '50 by means of an electrically actuated clutch I22 whichis controlled by a contact element I24 oi the pushbutton switch I32.

In the operation of the device for calibrating the variabie-speed-ratiotransmission device 24,

the circuit control element I48 is moved to the open circuit positionand the circuit control ele ment I14 is moved to the closed circuitposition. The pushbutton switch I32 is then operated and this will causethe deenergization of the clutches 92 and 94 by moving the contactelement I30 to open circuit position and the energization of the clutchI82 by moving the contact element I84 to closed circuit position. Theoperating wheel 82 may now be actuated to make the disc I40 appear tostand still thus indicating that the speed ratio of the transmissiondevice 2% is the same as the speed ratio of the gears I66-i@8. Theindication on the scale 90 may thus be verified since the speed ratio ofthe gears I66I6d is known, or in calibrating this scale 80 theindication may be marked on the scale. In a similar manner othersettings of the transmission device 24 may be calibrated or verified bythe selective actuation of the circuit control-element I16 and I12 torender the interrupters I52 and I54 operative at their respective knownspeeds. As many of the interrupter devices as desired may be providedsothat as many points on the scale 90 as desired may be calibrated orverified.

In the embodiment of the invention shown in Fig. 2 in which likereference characters designate the corresponding elements common to theembodiments of the invention of Figs. 1 and 2, the strip moves throughthe mill in the direction indicated by the arrows and the motion of thestrip on the delivery side of the mill is transmitted from the rollerIII through the synchrot'ie system 52, the shaft 18, the clutch 92, andthe shaft 96 to one side of the speed-differential responsive device 22,and the motion of the strip on the entrance side of the mill istransmitted from the roller I2 through the synchro-tie system 52, a setof gears 34, the shaft 65, the clutch 24, and the shaft 98 to the otherside of the speed-dinerential-responsive device 22. The shaft I whosespeed is governed by the speed-differential of the shafts 96' and 98 isconnected to drive a generator 26 whose field winding is connectedthrough a rheostat I88 to any suitable source of direct cur ent power.Thus, it will be seen that the output potential of the generator 25 willvary with the diflerence in the speeds in the strip on the entry anddelivery sides of the mill. The set of gears 24 has a very small speedratio, such as approximately 98/100. This provides a normal potentialbias or the generator 22 so that it will have an appreciable potentialat the low degrees of reduction of the strip I4 thereby assuring moreaccurate readings on the indicator 22. This functioning of the gear unit24 is best illustrated by the graph of Fig. 3 in which the outputpotential of the generator 22 is plotted as a function 01' the per centdraft taken by the mill, comparing the conditions of the operation withthe gear unit-24 and without the gear unit 24. By reference to Fig. 3,it will be seen that without the use of the gear unit 24 with a smallper cent draft such as .5% the potential output of the generator 22 willbe approximately 5 volts. With the gear unit 24 in use, however, with adrait of .5% the potential output of the generator 25 will beapproximately 15 volts. Since electro-responsive indicators andregulators cannot be accurately calibrated and controlled at lowerpotentials, it will be seen that this potential bias supplied by the useof the gear unit 24 will result in much more accurate draft indicationsand regulation at the lower percentages of draft.

In order to compensate this system for variations in speed of the mill,the generator 44 is connected in circuit with the generator 36 and isdriven at a speed inversely proportional to the speed 0! the mill sothat its output potential will be inversely proportional to the speed ofthe mill. The generator 44 is provided with a field winding which may beenergized by direct current energy as indicated. The generator 44 isdriven through a differential mechanism 48 having asun-gear I94 drivenby the roller I2 and a sungear I95 driven at a constant speed by themotor 6. The diflerential speed as determined by the differentialmechanism 48 is transmitted through the gears I 98 and 200 and a shaft202 to the generator 44. The regulator 40 is connected bymeans o1conductors 2G4 and 206 to control the energization of the armature I22of the drive motor for the screw-down actuating mechanism 28.

The tensioning device 42 comprises a roller 202 mounted upon a crank mwhich is actuated to cause the roller 202 to deflect the strip I4 fromits normal pass line and thereby subject it to tension. The crank 2I0may be actuated by a fluid operating piston and cylinder arrangement2I2, the force applied by the piston being automatically regulated by asuitable electrically adjustable regulating valve 2, The amount oftension applied to the strip may be controlled by the regulator 40through conductors 2 I6 and 2I8 connected to the pressure regulatingvalve H4. The deflection of the strip by the tension device 42 may bekept within predetermined limits by means of a regulator 220 actuated inaccordance with the deflection of the strip by the tensioning device 42and acting upon the field winding 222 of the drive motor 224 for theroll stand 226 to regulate the amount of slack in the strip by controlof the speed of the roll stand 225. amount of strip reduction for agiven screwdown setting is a function of the tension applied to thestrip as it passes through the reducing rolls.

In the operation of the embodiment of the invention shown in Fig. 2, itwill be seen that the output pct: ntial of the generator 26 will beprofore, should be a measure of the reduction of the strip in the mill.However, should the speed of the mill vary, the output potential of thegenerator 3% would not be a true indication of the draft taken in themill, although it would be a true indication of the difierence in thespeeds in the strip on the entry and delivery sides of the mill. Withthe generator 13% operating at a speed proportional to the diflerencebetween the speeds of the strip on the entry side of the mill and themotor it, this output potential will be inversely proportional to thespeed of the mill. Thus for a given per cent draft the potential of thegenerator M will tend to compensate for the variations in the outputpotential of the generator it due to variations in the speed of the milland the sum of the output potentials oi the generators 3t and 6a will bean accurate indication of the draft taken inthe mill. This functioningof the generator 56 to compensate for variations in the speed of themill is illustrated by the graph in Fig. 4. It will be seen that for agiven per cent draft the potentials of the generators 36 and M will beadjusted by means of the rheostats ittl and 592 so that the sum of theirpotentials throughout the range of variation of the strip speed will beconstant. For 1% draft, for instance, the potential of the generator 36will be adjusted to vary from zero to 20 volts and the potential of thegenerator M will be adjusted to vary from 20,volts to zero through therange of strip speed from zero to the speed attained at the maximumspeed of the mill. Thus the sum of the potentials of generators 86 andit will always equal 20 voltswhen 1% draft is being taken regardless ofthe mill speed. Any variation of the draft, however, with the variationof the speed of the mill will cause a variation of the output potentialof the generator and this variation will be observed on the indicator 38and will actuate the. regulator id to vary the screw-down setting of therolls and to'vary the tension applied to'the strip by the tension device52. a

.When it is desired to produce a difierent percentage draft in the mill,the rheostats E88 and 92 will bevaried to produce the potentialindicated by the graph of Fig. 4 corresponding to the desired draft, andthe regulator will be adjusted to maintain the screw-down setting andtension on the strip which will produce the desired draft.

It is to be understood that tension devices such as the tension device62 described in connection with" the embodiment of the invention shownin Fig. 2 may be applied on both sides of the mill stand in the systemof Fig. 2 and that the regulator may be made to control the tension ofthe strip only or the screw-down setting only or both the tension andthe screw-down setting, either together or in sequence, and that thetension device ma be used either with thescrewdown setting control ofthe embodiment of the invention of Fig. 1 or instead of the screw-downsetting control, and that the tension device may be used on either orboth sides of the mill stand in the system of Fig. i.

It will be seen that I have provided a draft control system for a striprolling mill which functions to indicate and control the draft taken o ale th of material as it passes th the 75 engaging the strip and anelectromagnetic clutch mill, which shall be adjustable to take anydesired predetermined draft, which shall automati cally compensate forvariations in the mill speed, which shall be simple and emcient inoperation and shall be inexpensive to manufacture, install,

- and maintain.

ments shown herein being merely illustrative of the principles of myinvention.

I claim as my invention: 1. In a draft control system for a striprolling mill, means applying a tension to the strip as it passes throughthe mill, and means responsive to the difference between the entry anddelivery speeds of the strip for causing said tension applying means tocontrol the tension applied to the strip.

2. In a draft control system for a strip rolling mill, means applyingtension to the strip as it passes through the mill, a differential gearunit having its respective sun-gears connected to be actuated inaccordance with the entry and delivery speeds of the strip as it passesthrough the mill, and means responsive to the difierence between theentry and delivery speeds of the strip as measured by said diife'rentialgear unit for causing said tension applying means to control the tensionapplied to the strip.

3. In a draft control system for astrip rolling mill, means applyingtension to the strip as it passes through the mill, a mechanicalspeed-differential responsive device actuated in accordance with theentry and delivery speeds of the strip as it passes through the mill,and means responsive to the difference between the entry and deliveryspeeds of the strip as measured by said differential device for causingsaid tension the strip.

4-. In a draft control system for a strip rolling mill, in combination,means for controlling the screw-down setting of the reducing rolls ofthe mill, means applying tension to the strip as it passes through themill, a mechanical speed= differential responsive device actuated inaccordance with the entry and delivery speeds of the strip as it passesthrough the mill, and means responsive to the difference between theentry and delivery speeds of the strip as measured by saidspeed-differential device for controlling said tensioning means and saidscrew-down setting control means.

5. In a draft control system for a strip rolling mill, in combination, amechanical speed-differential responsive device, drive means including afor connecting the strip on the entry side oi the mill in drivingrelation with one side of said speed-differential responsive device,means including a roller engaging the strip and an electromagneticclutch for connecting the strip on the delivery side of the mill indriving relation with the other side of said speed-difierentialresponsive device, and circuit control'means responsive to theengagement of the strip with said rollers for eil'ecting theenergization of said clutches only when the strip is in driving relationwith both of said rollers.

7. In a draft control system for a strip rolling mill, in combination,two stroboscdpic members comprising a rotatable member and a circuitmake-and-break device, means connecting the strip on one side of themill in driving relation with one of said stroboscopic members, meansincluding a selectively adjustable variable speedratio transmissiondevice connecting the strip on the other side of the mill in drivingrelation with the other of said stroboscopic members, a light producingdevice disposed to illuminate said rotatable member, and meansconnecting said circuit make-and-break device to control theenergization of said lightproducing device, said variable speed-ratiotransmission device comprising speed-ratio adjusting means, andauxiliary makeand-break means connected to intermittently energize saidlight producing device at selectable frequencies that bear predeterminedratios to the speed of the driven side of said variable speedratiotransmission device, whereby the said speed-ratio adjustment indicatingmeans may be calibrated and its calibration verified.

8. In a draft control system for a strip rolling mill, in combination,two stroboscopic members comprising a rotatable member and a circuitmake-and-break device, means connecting the strip on one side. of themill in driving relation with one of said stnoboscopic members, meansincluding a selectively adjustable variable speed-ratio transmissiondevice connecting the strip on the other side of the mill in drivingrelation with the other of said stroboscopic memhers, a light producingdevice disposed to illuminate said rotatable member, means whereby saidmake-and-break device controls the energization of said light producingdevice, differential-speed responsive means responsive to the difierencebetween the speeds of said two stroboscopic mem bers, and meansresponsive to said difierentialspeed responsive means for varying thereducing forces applied to the strip.

9. In a draft control system for a strip rolling mill, in combination, aspeed-difierential responsive device, means connecting the strip on oneside of the mill in driving relation with one side means including avariable speed-ratio transmission device connecting the strip on theother side of the mill in driving relation with the other side of saidspeed-difierential responsive device, means responsive to saidspeed-difierential responsive device for controlling the reducing forcesapplied to the strip the mill, a strobo- .0! said speed-diflferentialresponsive device,.

transmission device comprising a manually operated speed-ratio adjustingmeans and means for indicating the adjustment of the speed-ratio,auxiliary stroboscopic control means for calibrating said speed-ratioindicating means and verifying its calibration, and means forsimultaneously rendering saidspeed-diflerential responsive deviceinoperative and rendering said auxiliary stroboscopic control meansoperative.

l0. Ina draft control system for a strip rolling mill, in combination, aspeed-differential responsive device, means connecting the strip on oneside of the mill in driving relation with one side of saidspeed-difierential responsive device, means connecting the strip on theother side or the mill in driving relation with the other side of saidspeed-differential responsive device, means for maintaining slightlydifierent ratios between the speeds of the strip on the two sides oi!the mill and the corresponding sides of the speeddiiierential responsivedevice, a generator, means for controlling the output potential of saidgenerator in accordance with the speed difierential measured by saidspeed-difierential responsive device, and means responsive to the outputpotential of said generator for indicating the draft taken by the mill.

11. In a draft control system for a strip rolling mill, in combination,a speed-differential responsive device, means connecting the strip onone side of the mill in driving relation with one side of saidspeed-difierential responsive device, means connecting the strip on theother side of the mill in driving relation with the other side of saidspeed-diflferential responsive device, means for maintaining slightlydifferent ratios between the speeds of the strip on the two sides of themill and the speeds of the corresponding sides of the speed-differentialresponsive device, a generator, means for controlling the outputpotential of said generator in accordance with the speed-differentialmeasured by said speeddifierential responsive device, and. meansresponsive to the output potential or" said generator for controllingthe reducing forces applied to the strip.

12. In a draft control system for a strip rolling mill, in combination,a speed-differential responsive device, means connecting the strip onone side oi the mill in driving relation with one side of saidspeed-differential responsive device, means connecting the strip on theother side of the mill in driving relation with the other side of saidspeed-differential responsive device, means tor-maintaining slightlydifferent ratios between the speeds of the strip on the two sides of themill and the corresponding sides of the speed difierential responsivedevice, a generator, means for controlling the output potential of saidgenerator in accordance with the speed-difierential measured by saidspeed-differential responsive device, means responsive to the outputpotential of said generator for indicating the draft taken by the mill,and means responsive to the speed of the strip on the entry indicatingmeans for variations in the operating speed of the mill.

of said speed-differential responsive device, means for maintaining aslightly difierent ratio between the speeds of the strip on the twosides of the mill and the corresponding sides of the speed-difierentialresponsive device, a generator,

means ior controlling the output potential of said generator inaccordance with the speech difierentlal measured by saidspeed-difierential

